Pembroke synchronous condenser enters service as grid stability provision changes

RWE’s new synchronous condenser at Pembroke is now supporting Britain’s transmission system, marking the first commissioning from a new wave of 16 machines being delivered through NESO’s Stability Pathfinder programme. NESO estimates that the stability technologies being introduced through the programme could save consumers up to £9 billion in system costs over the coming years.

Historically, inertia, voltage support and fault-current contribution arrived largely as a consequence of running big synchronous generators at coal, gas and nuclear power stations. The problem is, going forward inverter-connected generation, think wind and solar, will likely take a larger role in the amount of electricity that is produced in the UK, while synchronous generators will be running for shorter periods of time. 

So, what does that all mean? Well, if you’re running synchronous generation for shorter periods of time, it means that we can’t simply assume that the other services that we gain from them – inertia, voltage support and fault-current – will be available exactly when we need them. That’s because they were a ‘by-product’ of that type of generation, while inverter-based generation will need additional intervention that it can’t do on its own. 

That’s why Pembroke exists. It’s a way of separating the services that were previously a by-product of electricity generation, from the actual generation of electricity. It’s essentially a dedicated machine, dubbed a synchronous condenser, which has no prime mover and does not generate power. Its rotating mass contributes inertia to resist rapid frequency changes, while RWE says the unit can also inject or absorb reactive power to regulate voltage, contribute fault current and strengthen electrically weaker parts of the network.

It’s important to note the distinction, because it’s changing how the electricity network is being built in the future. Stability is becoming something that must be deliberately located, specified, modelled and commissioned rather than treated as an incidental characteristic of whatever generation happens to be running. That puts greater emphasis on voltage-control performance, fault levels, system-strength studies and evidence that an asset will deliver its contracted response under the required network conditions.

Of course, some may be reading this and thinking ‘synchronous condensers are not the only answer’, after all NESO says its Pathfinder programme also includes two grid-forming batteries connected to Britain’s transmission network in 2025. The problem is those two batteries remain the only ones taking part in that particular programme, as back in March, not a single battery system secured contracts in Stability Market Round 2. It was blamed on the fact that the submissions failed the technical assessments, but it was a big blow to battery technology – especially as synchronous condensers and open-cycle gas turbines secured 7.3 GVA of inertia contracts.

It’ll be interesting to see how the technology develops, however. There are still many more Pathfinder machines to commission, and the proof will be in the pudding as to whether they can meaningfully replace the previous method of providing inertia, voltage support and fault-current contribution. We’ll also soon know whether more grid-forming batteries will be joining the existing two, with the winning contracts of Stability Market Round 3 set to be announced in October 2026.